1. Field of the Invention
The present invention is directed to a projection display for sequentially illuminating and imaging at least two light valves, and more particularly, to projection display employing a total-internal-reflection prism and a rotating wheel having two surfaces for simultaneously illuminating both light valves with different colors.
2. Discussion of the Prior Art
Typically, conventional projection displays have three light valves or spatial light modulators (SLMs). To reduce cost and complexity of projection displays, field-sequential color mode is used, where only one SLM is used to produce a full color image, instead of three SLMs, one for each of the primary colors, red (R), green (G), and blue (B).
In projection displays using the field-sequential color mode method, a full color image is produced using a single SLM by time-averaging the three primary colors. The single SLM is illuminated sequentially by the three RGB colors, one color at a time. For example, the single SLM is first configured with the red image data and exposed to red light for one-third of a frame time. Next, the single SLM is configured with the green image data and exposed to green light for the second third of the frame time. Finally, the single SLM is configured with the blue image data and exposed to blue light for the final third of the frame time. The full color image is the time average of three individual color sub-frames. This sequential illumination of the single SLM requires the SLM to operate at three times the illumination or update rate of each color. For example, if the full color frame is renewed at 60 Hz, then the SLM must operate at 180 Hz.
Although using only a single SLM, instead of three SLMS, reduces cost and complexity of displays, such conventional single SLM displays have several disadvantages. First, the SLM must operate at higher frequencies, typically three times the normal video frame rate. Second, conventional single SLM displays are relatively inefficient since only one color at a time is used, thus discarding 2/3 of the light from a white light source. The efficiency is actually worse than 1/3, since part of the frame time must be reserved for electronic data addressing and for a period of time required for SLM stabilization, referred to as dead-time. For example, because the response time of the liquid crystal (LC) material available for each color of the single SLM displays can be many milliseconds, this dead-time can become half or more than half the frame time which is 1/180 seconds (or 5.6 msec), for example.
As shown in FIG. 1, instead of a single SLM, a projection system 10 has two SLMs 12, 14 for enhancing the performance of the field-sequential color mode of operation. Such a projection display 10 using two SLMs is described in U.S. Pat. No. 5,517,340 to Doany et al., where enhancement is achieved by alternately illuminating the two SLMs 12, 14 using a color wheel 16. In this case, only one SLM is illuminated at a time. This allows a half-frame time to reset the data and the liquid crystal response, and stabilize each one of the two SLMs, thus eliminating the dead-time needed for conventional displays using a single SLM in the field-sequential color mode. Rather than operating at 180 Hz, the two SLMs operate at 90 Hz each to produce a 60 Hz combined image video rate. Each SLM still has a 50% dead time, however, the combined two SLM system 10 has no dead time, since one of the two SLMs 12, 14 is always illuminated. Although the two SLM configuration 10 eliminates the requirement of a dead-time, the spectral efficiency is still only 1/3.
As shown in FIG. 1, a light source 18 provides white light to a polarizing beam splitter (PBS) cube 20. The PBS 20 has a coating that reflects one of the linear polarizations of light, such as the s-polarization, and transmits the other polarization, e.g., the p-polarization. The PBS cube 20 illuminates the two SLMs 12, 14 with light having different polarizations. For example, the PBS cube 20 illuminates one SLM 12 with s-polarized light and illuminates the other SLM 14 with p-polarized light. The PBS cube 20 recombines images reflected from the two SLMs 12, 14, to form a color image which is projected by a projection lens 22 onto a projection screen 24.
The two SLM projection display 10, which operates in the field-sequential color mode, has 50% duty cycle (or 50% ON-time and 50% OFF-time) for each SLM. FIG. 2 shows a timing diagram 100 of the conventional two SLM projection display 10 shown in FIG. 1. As shown in FIGS. 1 and 2, the color wheel 16 has six segments and rotates a full revolution in a time period 110 of 1/30 seconds (33.3 ms).
The six segments of the color wheel 16 include two red, green and blue (RGB) frames of alternating polarizations between successive colors, shown in FIGS. 1 and 2 as R-s, G-p, B-s, R-p, G-s and B-p, where s and p are the two linear polarizations of light, which are orthogonal to each other. The SLM 12 that receives s-polarization is shown as SLM-1 in FIG. 2, while the other SLM 14 that receives p-polarization is shown as SLM-2.
As shown in FIG. 2, the overall system cycles through an RGB frame twice in the 33.3 ms period 110, as the color wheel 16 rotates through its six segments, which correspond to two RGB frames. Thus, one full color frame is cycled every 1/60 seconds (16.67 ms), or at 60 Hz, shown as time period 115 in FIG. 2.
Instead of an 180 Hz SLM operation rate for a single SLM projector to provide a 60 Hz overall video rate, each SLM 12, 14 of the conventional two SLM projector 10 (FIG. 1) operates at only 90 Hz, despite an overall video rate of 60 Hz. Each SLM 12, 14 has a refresh cycle time 120 of 1/90 seconds (11.1 ms), and is "ON" for 1/180 (5.556 ms) and "OFF" or "dead" for 1/180 (5.556 ms), shown in FIG. 2 as reference numeral 130. The two SLMs 12, 14 operate at opposite phase relationship, where one SLM is "ON" while the other SLM is "OFF".
Under proper conditions, (e.g., a low-voltage liquid crystal material, very small liquid crystal cell gaps, and using higher operating voltages), the liquid crystal (LC) response time can be reduced below 2 ms. The time allotted by the conventional two SLM projection device 10 is much longer than 2 ms, such as the 5.356 ms period 130, shown in FIG. 2. Thus, the conventional two SLM projector 10 of FIG. 1 is inefficient as it allows more OFF time then the required 2 ms LC response time.
Further, the conventional two SLM projector 10 of FIG. 1 illuminates only one SLM at a time. Thus, an average 50% duty cycle must be maintained and cannot be exceeded, which limits the efficiency of the conventional two SLM projector. Accordingly, there is a need for a full color projection display which is highly efficient.
Moreover, the conventional two SLM projector 10 of FIG 1 has a limited contrast ratio. Improvements in the coating of the PBS cube 20 increases the contrast ratio. However, improving the contrast ratio based on improving the coating of the PBS cube 20 is difficult.
To achieve greater than 100:1 contrast ratio, the PBS coating must operate efficiently in reflecting more than 99% of the s-polarized light, while transmitting more than 99% of the p-polarized light over the entire visible spectrum, and over a reasonable range of illumination angles (e.g. .+-.10 degrees). The performance of currently available optical coatings is not adequate to achieve these goals. Although greater than 99% s-reflection is possible, greater than 90% p-transmission at large angles (on the order of 10 degrees) is difficult.
Typical optical systems using reflective SLMs and a PBS must include an absorbing clean-up sheet polarizer film in the reflection path of the PBS to ensure adequate contrast ratio performance. In the conventional two SLM projector 10 of FIG. 1 using the color wheel 16, both polarizations are used simultaneously. Therefore, it is not possible to include a clean-up polarizer. Accordingly, there is a need for a full color projection display with high efficiency and contrast that allows use of a clean-up polarizer, and does not require an improved PBS coating.